Smart Transactions UI and nonce editing confusion on Infura vs alternative RPC URLs

### What is this about?

STX transactions are supported only when using Infura. When users switch to an alternative RPC URL, STX functionality reverts to a standard send transaction without any explicit notification, which might not be immediately clear to the user. This can lead to confusion as the user interface changes unexpectedly.
Additionally, users have reported an inability to edit the nonce value when connected to Infura, a feature that was accessible on other RPCs. This behavior aligns with the expected functionality when STX transactions are enabled, yet the rationale behind this limitation was not apparent to users.
To enhance clarity and user experience, it would be beneficial to introduce a notification or alert informing users that STX functionality is exclusive to certain RPCs.

### Scenario

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### Design

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### Technical Details

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### Threat Modeling Framework

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### Acceptance Criteria

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### Stakeholder review needed before the work gets merged

- [ ] Engineering (needed in most cases)
- [ ] Design
- [ ] Product
- [ ] QA (automation tests are required to pass before merging PRs but not all changes are covered by automation tests - please review if QA is needed beyond automation tests)
- [ ] Security
- [ ] Legal
- [ ] Marketing
- [ ] Management (please specify)
- [ ] Other (please specify)

### References

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Start by tracing the Smart Transactions UI behavior when connected to Infura versus an alternative RPC URL, including how nonce editing is enabled or disabled. Done should include a clear notification when STX functionality is unavailable on the selected RPC and an understandable explanation of the nonce limitation.

Written by the indexing model from the issue text.